The present invention relates to methods and apparatus for manufacturing hollow articles of thermoplastic material, for example, polyester, polypropylene, polyvinyl chloride, nylon, from injection molded or compression molded preforms which are inflated in their softened state until they take the shape of the desired articles.
Typical examples of the prior art are U.S. Pat. Nos. 3,048,889, 3,339,231, 3,412,186, 3,664,798, 3,776,991, 3,842,149 and 4,295,811. In addition, attention is drawn to a typical modern injection blow molding machine disclosed in Sumitamo Heavy Industries, Ltd.'s brochure M202 E 81 entitled Nepiomat 30/120, 330/150.
It is known in the prior art to effect such a process by pressing the edge of a preform in the softened condition onto the neck of a hollow blow mold which has the shape of the desired article. The preform is then blown or inflated in the softened condition within this mold, starting from the edge thereof, until it completely touches the entire interior of the blow mold, and on contact with this wall, the distended preform cools instantaneously, thereby permanently assuming the shape of the mold, i.e., the desired article.
By the term "softened" state, or condition, or phase, is meant the state of the material intermediate the rigid and fluid states, a state in which the material can be mechanically worked without rupturing, for example, a state in which the material is capable of being expanded by blowing. Both thermoplastic and thermoset plastics are capable of entering the "softened" phase. As to thermoplastics, they typically enter the softened state upon being heated and reenter the hard or rigid state upon being cooled, and are capable of thus cycling between the softened and rigid states a number of times. In contrast, thermoset resins typically pass through this "softened" phase only once, immediately after initial formation, and thereafter, they irreversibly set in the rigid condition.
In such blow molding operations, for high mechanical strength properties of the blown container (such as impact and stress resistance), it is of great importance with certain resins that a high level of biorientation of the resin of the container be achieved. This requires close temperature control of the resin of the preform.
For reasons of production speed and in order to prevent the soft preform from being left supporting its own weight between the injection and blowing phases of the process, the blowing head is generally incorporated in the core upon which the preform or parison is formed. The core containing this blowing head is then conveyed to the entrance of the blow mold.
In the injection blow molding apparatus presently being used, such as the Piotrowski system disclosed in Piotrowski U.S. Pat. Nos. 3,339,231 and 3,412,186 above referred to, in which the preform remains with the core and is transferred to the blow station to make a container, controlled temperature conditioning, which is essential for obtaining optimum biorientation levels in the finally blown container, is difficult to predict or achieve, especially in the following situations:
a. When the preform has a thickness from about 1-5 mm, for example, 2.5 mm, the final blown container can have a wall thickness of about 0.25 mm or less. This is especially so when the blow ratio between the preform wall and the blown container is fairly high, such as 10 to 1, which is preferred for high biorientation levels with some resins.
b. When using certain resins, such as polyester teraphthalate, polypropylene, polyvinyl chloride, nylon, etc., the temperature is very difficult to control on a predictable basis in present injection blow mold equipment in which the core travels with the preform to the blow station. These resins, in their hot softened state or at their optimum biorientation blowing temperatures, tend to adhere or stick unpredictably to parts of the metal core when blow air is introduced for blowing the container. This condition causes poor material distribution in the walls of the container, and in extreme cases, folds, blowouts, pleats and other distortions.
c. In the manufacture of consistently controlled bioriented containers for use as pressure vessels, i.e., beverage, beer and other carbonated drinks, made from polyester teraphthalate bioriented containers, orientation levels in the walls of the containers have to be predictable and consistent, and this can only be accomplished by fairly precise control of the preform temperature at the time it is blown to form the container. This predictability in control is presently not readily and consistently available on equipment being used in the marketplace.
In present injection blow molding apparatus, temperature control of the cross section of the preform wall is difficult to achieve. The core which is carrying cooling fluid tends to overcool the inner layer of the preform which is in direct contact with the core. The outer layer of plastic of the preform does not cool at the same rate and cools partly by convection to ambient air. The only primary cooling that the outer layer gets is when it is stationary in the injection cavity. Heat transmission from the outer layer of the preform to the core is slow due to the poor heat transfer characteristics of plastics in general. This condition causes the temperature gradient through the cross section of the preform wall to be very steep, especially in thicker preforms which are required for high blow ratios. High blow ratios are required to obtain high biorientation levels in the blown container walls.
To get maximum utilization of plastic in a container, it is necessary to maximize the mechanical properties of the walls of the container through biorientation at the optimum orientation temperature for the particular resin being used.
The part of the cross section of the preform which is nearest to optimum orienting temperature develops the best mechanical properties when blown into a container. Consequently, the portions of the plastic in the preform in higher and lower temperature bands tend to have unsatisfactory properties.
The process temperature window for optimum orientation levels in orientable resins is fairly small. In view of this, in order to obtain the best results, the entire cross section of that portion of the preform that needs to have optimum mechanical properties should be as close as possible to the precise biorienting temperature at the time it is blown. There should be a minimum and, ideally, no temperature gradient.
It is an object of the present invention to control the temperature conditioning of preforms and to achieve this with an economically effective production system.
The present invention provides a hollow preform of plastic material for the blow molding of a container comprising an open end portion, a hollow body portion and an end closure, said open end portion having an inner surface defining an opening of constant cross section and said body portion having an interior surface defining an opening extending from the opening of said open end portion with an ever decreasing cross section to said end closure.
The present invention also provides apparatus for the blow molding of containers comprising a core for supporting a thermoplastic preform during molding thereof and for supplying fluid to the interior of the preform for blow molding thereof, said core having a sealing portion of constant cross section and a body portion extending from the cylindrical portion with an ever decreasing cross section to a core end portion.
The apparatus preferably also comprises a split clamp means adapted to clamp a preform open end portion in sealing contact with said constant cross section sealing portion, said clamp means and said core being adapted for relative movement to space a portion of a preform, when on the core, from said core while said sealing contact is maintained.
The apparatus further includes an injection blow molding press including injection molding and blowing stations and having a fixed platen, a movable platen and a turret for mounting the preform core, with a neck ring split or clamping means arranged to shift the position of the preform with respect to the preform core in different positions of the press in order to make possible improved temperature conditioning and control of the preform. This provides for accuracy of biorientation temperature control, notwithstanding shifting movement of the core between injection and blowing stations.
The apparatus of the invention is also adapted to the use of a multiplicity of cores sequentially brought into cooperation with the preform injection and blow molding stations, thereby providing automatically for the carrying out of the following steps:
a. molding a preform onto a core, said preform having an open end portion disposed in sealing engagement with a surface of constant cross section on said core, and a hollow body portion having a closed end;
b. moving said preform and said core relative to one another to space said body portion from said core while maintaining said sealing contact;
c. achieving a desired temperature condition of said preform, while spaced from said core, for blow molding to produce a bioriented thermoplastic container; and
d. blow molding the temperature conditioned preform to produce said container.
It is also an object of this invention to improve the distribution of the material in the walls of the hollow containers being blown, this being accomplished according to the present invention by the employment of an element, herein referred to as a stretch rod, mounted in the core and arranged for projection from the tip of the core to the bottom of the preform and of the article as it is being blown, thereby guiding the bottom wall of the container during the blowing operation.
In arrangements of the invention employing the stretch rod, the stretch rod is also associated with a valve in the passage through which air may be introduced into the preform, such valve providing not only for introduction of air to diminish the tendency for a vacuum to develop causing distribution of the preform on the core when the preform is shifted on the core, but also to subsequently introduce the blowing air, to effect blow molding of the article.